Hypercholesterolemia and Coronary Artery Disease
Coronary artery disease (CAD) is the leading cause of death, the most common form of heart disease and the most common cause of sudden death in the western world. Clinical and epidemiological evidence have established a clear link between elevated serum cholesterol and CAD. Within apparently healthy populations, there is an exponential relation between serum cholesterol and coronary risk. In middle age, the risk of CAD increases by 2 to 3% for each 1% increase in cholesterol levels.
An estimated 107 million American adults have total serum cholesterol levels of 5.18 mmol/l (200 mg/dL) and higher. Of these, approximately 37 million have levels of 6.22 mmol/l (240 mg/dL) or above. In adults, total cholesterol levels of 6.22 mmol/l or higher are considered high risk for cardiovascular related events while levels between 5.18 and 6.22 mmol/l are considered borderline high risk. According to the recommendations of the National Cholesterol Education Program's (NCEP) the primary objective of any therapy is the lowering of LDL Cholesterol levels (Third Report of the NCEP Expert Panel 2002). New guidelines now consider other risk factors such as age, family history, smoking, hypertension, low HDL, and diabetes mellitus, in estimating cut-off levels of cholesterol requiring intervention. LDL goals in primary prevention therefore depend on a patient's absolute risk for CAD related events in the short term or long term. Currently, according to the recently revamped recommendations of the NCEP, an additional 36 million US citizens should be treated for high cholesterol. Currently, less than half of patients who qualify for lipid modifying treatment are receiving it and only a third of treated patients are achieving their LDL cholesterol goal.
Pathogenesis of Atherosclerosis
The involvement of elevated LDL cholesterol in atherosclerosis and CAD is well documented. Atherosclerosis is initiated by the retention of apolipoprotein B-containing lipoproteins (e.g. LDL cholesterol) in the arterial wall. Over time, lipoproteins retained in the arterial wall become modified (i.e. aggregated and oxidized) and elicit a cascade of biological responses that develop into a maladaptive inflammatory response (Tabas et al. 2007). In particular, monocytes enter the subendothelium, differentiate into macrophages and ingest the retained modified lipoproteins to become cholesterol-laden foam cells. Eventually, inflammatory cells enter the lesions and help contribute to the aforementioned maladaptive inflammatory response, a process accelerated by amplified retention of lipoproteins in established lesions. A process mediated by cytokines and growth factors causes smooth muscle cells to migrate and form a collagenous fibrous cap (mature atherosclerotic plaque), most likely as a scar-like response to wall off the lesion (Tabas et al. 2007). However, as the lesion progresses, macrophages die, resulting in areas of necrosis containing extracellular debris, cholesterol crystals, proteases and thrombotic material. At this point, fibrous cap thinning, plaque eruption or erosion may occur, potentially leading to acute thrombotic vascular events such as myocardial infarction and stroke.
High-density lipoproteins play a key role in “reverse cholesterol transport”, a pathway by which excess cholesterol is removed from extrahepatic cells and returned to the liver for excretion from the body. In the peripheral tissues, HDL is believed to remove cellular cholesterol through a variety of mechanisms including interaction of HDL apolipoproteins with cell-surface binding sites or receptors (Tall, 1998). The action of lecithin-cholesterol acyltransferase (LCAT) converts the absorbed cholesterol into cholesterol esters and in turn can increase the absorption capacity of HDL. Upon return to the liver, cholesterol may be metabolized into bile salts and excreted from the body. LDL and HDL cholesterol are the major factors in maintaining the cholesterol balance of the body and a high ratio of HDL to LDL correlate well with a lower incidence of CAD in humans.
High serum triglyceride levels are similarly a risk factor for atherosclerosis and CAD. Specific reasons for this include the increased production of atherogenic chylomicron and VLDL remnants, the inverse relationship present between serum triglyceride and HDL, the possible resultant increase in LDL attributable to remnant-reduced hepatic LDL-receptors as well as the formation of more dense and, therefore, more atherogenic LDL, and to the interaction between serum triglyceride and the fibrinolytic/coagulation system. Because of the multiple links between elevated triglyceride levels and risk for atherosclerotic cardiovascular disease, screening for hypertriglyceridemia is important when determining a patient's risk for atherosclerotic cardiovascular disease.
Immune Responses in Atherosclerosis
The pathogenesis of atherosclerosis is believed to include dyslipidemia, vascular endothelium dysfunction, and a chronic inflammatory process. Several mediators have been shown to be involved in intercellular signaling in atherosclerosis, including small molecules such as nitric oxide, lipid mediators such as eicosanoids and sterols and cytokines. Inflammation is mediated by cytokines, glyco-proteins involved in cell to cell signaling, which are produced by macrophages and dendritic cells in the epithelium in response to an antigenic or foreign body stimulus. The immune response is implicated in the formation of early fatty streaks, when the endothelium is activated and expresses chemokines and adhesion molecules leading to monocyte/lymphocyte recruitment and infiltration into the subendothelium. It also acts at the onset of adverse clinical vascular events, when activated cells within the plaque secrete matrix proteases that degrade extracellular matrix proteins and weaken the fibrous cap, leading to rupture and thrombus formation. Recently, toll-like receptors (TLR) on the surface of the gastrointestinal epithelium have been linked to the induction of an inflammatory response, helping to initiate the start signal for the production of pro-inflammatory cytokines (Tobias and Curtiss, 2007).
Specific emphasis is placed on the contribution of pro- and anti-inflammatory cytokines to pathogenic (innate and adaptive) and regulatory immunity in the context of atherosclerosis. Cytokines can be differentiated by those with an essentially pro-inflammatory mode of action, including tumor necrosis factor (TNF-alpha), interleukin-12, IL-18 and interferon gamma from those with anti-inflammatory mode of action, including IL-4, IL-10, IL-13 and the endogenous IL-1 receptor antagonist IL-1ra. In response to the local milieu of cytokines, CD4+ cells differentiate into the Th1 (pro-inflammatory) or Th2 (anti-inflammatory) lineage. Among the principal inducers of the Th1 and Th2 cells are IL-12 and IL-10, respectively. Cytokines involved in the Th1 process include IL-2, IFN-gamma and TNF, while those involved in the Th2 process include IL-3, IL-4, IL-5, IL-6, IL-10 and IL-13. Over 30 major members of the interleukin family have been identified, the majority of which play a role in atherogenesis. Specifically, they have been attributed to primarily anti-atherogenic (IL-1ra, IL-9, IL-10, IL-11) and pro-atherogenic (IL-1, IL-2, IL-6, IL-18) properties. Modulating these interleukins represent the most readily applicable approach to immunotherapy in atherosclerosis. It is believed that gut bacteria initiate an inflammatory response when epithelium TLRs recognize non-commensal microbial motifs and this cytokine signal may translate to increased risk of atherosclerosis. The corollary of this response is that commensal microflora are required to maintain gut homeostasis through the recognition of their non-inflammatory motifs by TLRs. Recent research has shown that pro-inflammatory cytokines produced in the gut can be greatly decreased by delivering commensal bacteria (Lactobacillus acidophilus) delivered free in saline or in fermented milk (Urbanska et al. 2009). This research showed that L. acidophilus decreased IL-6, IL-12, TNF-alpha, and IFN-gamma levels when administered orally in saline and in fermented milk (only IL-6 data was published) (Prakash and Urbanska 2007).
In addition to pro- and anti-inflammatory cytokines, high sensitivity C-reactive protein is arguably the most important serum inflammatory marker of coronary risk. Recent research suggests that patients with elevated basal levels of CRP are at an increased risk of cardiovascular disease as well as diabetes, and hypertension. A clinical study of 700 nurses showed that those in the highest quartile of trans fat consumption had blood levels of C-reactive protein that were 73% higher than those in the lowest quartile (Lopez-Garcia, 2005). Others have shown that CRP can exacerbate ischemic necrosis in a complement-dependent fashion and that CRP inhibition can be a safe and effective therapy for myocardial and cerebral infarcts (Pepys et al. 2006).
Metabolic Syndrome
Dyslipidemia, atherosclerosis, and chronic inflammation are connected to other degenerative diseases through the metabolic syndrome. Metabolic syndrome is characterized by a group of metabolic risk factors in one individual and increases the individual's risk of developing atherosclerosis, cardiovascular disease, cerebrovascular disease and diabetes. This constellation of signs and symptoms affects one in five people, and prevalence increases with increasing age. Some studies estimate the prevalence in the USA to be up to 25% of the population (Ford et al., 2002). Symptoms and features include: Fasting hyperglycemia—diabetes mellitus type 2 or impaired fasting glucose, impaired glucose tolerance, or insulin resistance; High blood pressure; Central obesity (also known as visceral, male-pattern or apple-shaped adiposity), overweight with fat deposits mainly around the waist.
Non-Alcoholic Fatty Liver Disease (NAFLD)
Non-alcoholic fatty liver disease (NAFLD) is considered to be a hepatic manifestation of the metabolic syndrome. NAFLD is defined as fatty inflammation of the liver when this is not due to excessive alcohol use. NAFLD is strongly associated with obesity, dyslipidaemia, insulin resistance (IR) and type II (non-insulin dependent) diabetes mellitus. NAFLD covers the full spectrum of metabolic fatty liver disorders, particularly when histology is undefined. NAFLD can manifest as simple steatosis (fatty liver), at the most clinically indolent extreme, or can progress to steatosis with inflammation or fibrosis, in which case it is termed NASH. However, even stable forms of NAFLD may carry as yet unidentified morbidity since fatty liver typically functions less efficiently than non-fatty liver. NASH likely represents an intermediate stage characterized by steatosis with lobular inflammation. NAFLD is known to affect 10-39% of the general global population with an average incidence of 20% (Angulo 2002).
There are several risk factors associated with NAFLD. These factors include common life conditions and diseases such as obesity, hyperglycemia, type 2 diabetes mellitus, and hypertriglyceridemia. In addition, NAFLD is strongly associated with central obesity and visceral adiposity. Genetic and racial factors are also associated with NAFLD/NASH. This disorder will therefore contribute substantially to the burden of chronic liver disease in coming decades.
Treatment and Prevention of Hypercholesterolemia and Dyslipidemia
Methods for lowering cholesterol levels in humans involve dietary management, behaviour modification, and exercise and drug therapy. Dietary intervention alone is insufficient for most individuals. Studies show that complete elimination of dietary cholesterol and limiting fat content to less than ten percent of the daily caloric intake results in only a four percent regression of atherosclerotic plaques after five years when combined with stress management and aerobic exercise (Ornish et al. 1990).
Additional dietary options for LDL cholesterol lowering have been proposed, including soluble fibres, plant sterols and stanols and soy protein. Recent reports indicate that soluble forms of dietary fibre at 5-10 g per day can reduce LDL cholesterol by approximately 5% (Third Report of the NCEP Expert Panel 2002). Little, no, or inconsistent effects have been reported in regards to HDL cholesterol; however, it appears that modulation of cholesterol and bile metabolic pathways may be required as much evidence from studies that attempt to lower dietary intake or increase cholesterol catabolism result in decreases in HDL unless used in combination with cholesterol lowering medication that affects liver enzymes. Furthermore, insoluble fibre has not been shown to significantly affect circulating cholesterol levels. Animal and human studies show that plant stanols and sterols reduce plasma total cholesterol and low density lipoprotein (LDL) cholesterol levels. Data has shown that plant-derived sterol and stanol esters at dosages of 2-3 g/day decrease LDL cholesterol levels by 6-15% with no significant change in triglyceride or HDL cholesterol levels (Hallikainen and Uusitupa, 1999). Again, often studies that show no decrease in HDL or an non-statistically significant decrease in HDL have included patients on cholesterol lowering medication that alters liver enzymatic pathways such as Statins. Soy protein included in a diet low in saturated fatty acids and cholesterol has been shown to lower LDL cholesterol by about 5%, however, dosage requirements are not well known (Jenkins et al. 2000).
Statins can significantly reduce endogenous cholesterol synthesis, through inhibition of HMG-CoA reductase, and upregulate low-density lipoprotein receptors in the liver, leading to reductions in LDL-C of 20-30%. The efficacy of statins has been thoroughly evaluated in a multitude of clinical trials (Pedersen et al. 1994). Statins, however, have been shown to exhibit rare, but potentially severe, side-effects. The most predominant of these are myopathy, which may evolve into life-threatening rhabdomyolysis, and polyneuropathy (Gaist et al. 2001; Gaist et al. 2002; Omar and Wilson 2002; Staffa et al. 2002).
Fibrate therapy has also been shown to offer long-term benefits in high-risk patients with low HDL cholesterol-high triglyceride dyslipidemia (Goldenberg et al. 2008). Fibrates, however, are also associated with a variety of adverse effects including increased risk of gall stones, myopathy and stomach upset (Sgro and Escousse, 1991).
Niacin has been used for quite some time now, at doses of 1-2 grams per day, to reduce triglycerides and lower LDL-C. Interestingly, vitamin B3 has been shown to increase HDL-C at these levels as well and has been prescribed to patients with low HDL-C who are at risk of suffering a cardiac event. Unfortunately, uncomfortable and severe side effects including facial and full body flushing are exhibited with regular use.
Bile acid sequestrants (BAS) have been used clinically since the 1960s for lowering of LDL cholesterol. Bile acid sequestrants have a low rate of compliance caused, in part, by gastrointestinal side effects (Probstfiled and Rifkind, 1991).
Probiotics
Probiotics have been reported to be associated with a range of clinically relevant health benefits. Various strains of lactic acid bacteria have been particularly well studied in humans and animals. Placebo controlled clinical trials have shown L. reuteri, L. rhamnosus GG, L. casei and S. boulardii to be effective in reducing the duration of acute diarrhea (Huang et al. 2002). L. rhamnosus GG administered to infants reduced the risk of nosocomial diarrhea and rotavirus gastroenteritis (Szajewska et al. 2001). Studies by Aso et al. revealed that L. casei Shirota increases the percentage of T-helper cells and NK cells in adult colorectal cancer patients and has a protective effect on the recurrence of superficial bladder cancer (Aso et al., 1995). In addition, select strains of lactobacilli have been shown to significantly suppress intestinal tumors by chemical mutagens (McIntosh et al. 1999). Lactic acid bacteria have been administered to prevent sepsis in patients with severe acute pancreatitis. A randomized study by Rayes et al. involving liver transplant patients revealed postoperative infections were significantly reduced by feeding live L. plantarum cells in comparison to standard antibiotic treatment (Rayes et al. 2002). As a means of preventing allergy, a randomized controlled study by Lodinova-Zadnikova et al. investigated the effect of at birth colonization with nonpathogenic Escherichia coli Nissle 1917 (Lodinova-Zadnikova and Sonnenborn 1997). Subjects inoculated with the E. coli strain showed significantly reduced colonization of bacterial pathogens as well as significantly lower incidence of allergies after 10 and 20 years in comparison with control subjects. Probiotics have also been used as treatment options for managing Inflammatory Bowel Diseases (IBD) such as Crohn's disease, ulcerative colitis and pouchitis.
L. reuteri is well-established as one of the most ubiquitous members of the naturally-occurring gut bacteria. Host-specific strains of L. reuteri have been documented to confer broad-spectrum protection from an assortment of microbial and chemical associated disease in humans and animals (Dobrogosz, 2005). However, traditional probiotic therapy involves administration of bacteria with the hope that some bacteria will survive the harsh gastric conditions and colonize the colon where the bacteria will reproduce and live indefinitely. Far fewer bacteria survive in the duodenum, jejunum or ileum because of factors such as acidity, immune response and bile concentration. Bacteria must be present in the duodenum or jejunum of the small intestine for lowering cholesterol and in particular bile acid.